Common rail fuel systems typically employ multiple injectors connected to a common rail that is provided with high pressure fuel. In order to efficiently accommodate the different combinations of injections at a variety of timings and injection amounts, the systems generally include a variable discharge pump in fluid communication with the common rail. One type of variable discharge pump is the cam driven, inlet or outlet metered pump.
A cam driven, inlet or outlet metered pump generally includes multiple plungers, each plunger being disposed within an individual pumping chamber. The plunger is connected to a lobed cam by way of a follower, such that, as a crankshaft of an associated engine rotates, the cam likewise rotates and the connected lobe(s) reciprocatingly drives the plunger to displace (i.e., pump) fuel from the pumping chamber into the common rail. The amount of fuel pumped by the plunger into the common rail depends on the amount of fuel metered into the pumping chamber prior to the displacing movement of the plunger, or the amount of fluid spilled (i.e., metered) to a low-pressure reservoir during the displacing stroke of the plunger.
One example of a cam driven, outlet metered pump is described in U.S. Patent Publication No. 2006/0120880 (the '880 publication) by Shafer et al. published on Jun. 8, 2006. Specifically, the '880 publication teaches a pump having a housing that defines a first pumping chamber and a second pumping chamber. The pump also includes first and second plungers slidably disposed within the first and second pumping chambers and movable between first and second spaced apart end positions to pressurize a fluid. The pump further includes a first cam having three lobes operatively engaged with the first plunger, and a second cam having three lobes operatively engaged with the second plunger to move each of the first and second plungers between the first and second end positions six times during a complete cycle of the engine. The pump additionally includes a common spill passageway fluidly connectable to the first and second pumping chambers, and a control valve in fluid communication with the spill passageway. The control valve is movable to selectively spill fluid from the first and second pumping chambers to a low-pressure gallery to thereby change the effective displacement of the first and second plungers.
Although the cam driven outlet metered pump of the '880 publication may effectively pressurize fuel for a common rail system, it may be problematic. In particular, during each stroke of each plunger, significant force is directed from the plunger back through the respective cams, through a cam gear arrangement, and to a crankshaft of the associated engine. Although these forces by themselves might be insufficient to cause damage to the cams or cam gear arrangement, when coupled with other opposing forces such as those caused by combustion of the fuel, a significant hammering affect on the cams and/or cam gear arrangement may be observed. For example, when injectors of the same common rail system inject fuel to initiate combustion within the engine, resultant forces acting on the pistons of the engine travel down the connecting rod of each piston, through the crankshaft in reverse direction to the pump initiated forces, and into the cam gear arrangement. When the pump initiated forces and the injection initiated forces overlap (i.e., occur at the same time), the resultant force can be significant enough to cause damage to the cam gear arrangement and/or the cams of the fuel pump. Further, the forces acting on the components of the fuel system add to the overall noise of the engine, particularly when there is an overlap in the pump and injection initiated forces.
The disclosed fuel pump is directed to overcoming one or more of the problems set forth above.